(coexisting on another dimensional plane)
When she saw her captain show mercy to Spock by allowing McCoy to treat him for injuries suffered in a fight with Kirk, Moreau deduced that Kirk was in fact not her own captain, but instead a counterpart from a different universe (our own). She used the Tantalus field to save Kirk from an assassination attempt by Sulu. She eliminated Sulu's henchmen from existence with the field. At first she wanted to go with Kirk back to the other universe, but the transporter was set for only four people. Along with Spock, she helped Kirk and his crew escape from the ISS Enterprise before they could be discovered and killed. (TOS: "Mirror, Mirror")
Background information Edit
In the story outline of "Mirror, Mirror" (dated 1 April 1967), Marlena Moreau was instead named "Anna" and was actually married to Kirk. She was described as "beautiful and servile [...] [and] is obviously well trained in how to treat her husband." 
In The Sorrows of Empire, Marlena marries Spock after he takes control of the Enterprise following the events in "Mirror, Mirror", and is his chief ally during his eventual rise to power as Emperor of the Terran Empire. She is killed along with Spock by forces of the Klingon-Cardassian Alliance in 2295. Her father's name is François Moreau (mirror) (β).
In William Shatner's novel Dark Victory, Moreau is shown in a photograph in the mirror Kirk's (now known as Emperor Tiberius) secret base, hanging from gallows along with several other members of the Enterprise crew outside Starfleet Headquarters on Earth.